[Macworld.com]

Scanner glass gunked up from the inside? Google and a Torx #10 screwdriver can help to give it a fresh face. [more]

[TxTom]

I've had to disassemble almost every new scanner I've ever owned...UMAX, Epson, Microtek. For some reason, you can see a definite film on the inside glass. Using glass cleaner or alcohol cleans the outer surface, but that foggy appearance has always shown up on brand new scanners I've gotten. It can only reduce the d-Max of your scans. Fortunately, dedicated scanners from these companies have been pretty easy to pop open.
Hint: cover the scanning mechanism with something lint free while you've got it exposed to the open room...any dust lands on that and you'll end up with long lines in your next scans.
BTW, the film comes off easily from the underside of the glass--why or how it gets there from the factory is beside me.

[MacTechAspen]

In reply to:

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the film comes off easily from the underside of the glass--why or how it gets there from the factory is beside me

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Chances are it is due to plasticizers. These are added to plastics to make them flexible and durable. The same film ends up on the inside of your car windshield from the dashboard.
Plasticizers made from phthalates are the most common, and the most likely to evaporate from the plastic. There are other options, and little by little plastic companies are opting to move away from phthalates.
Our sign shop has a real problem with these causing bubbles in vinyl lettering. In most of the world these dissipate very quickly and are not much of an issue. At 8000 feet, they tend to be more severe (due, perhaps to the reduced air pressure on the outside).
A fact of life, although a nasty one, these phthalates are a potential environmental concern, as well as a general pain.